Ross & Wilson Anatomy and Physiology in Health and Illness (201 page)

Figure 17.9 
Autosomal inheritance.
Example shows all possible combinations of tongue-rolling genes in children of parents heterozygous for the trait. T: dominant gene (tongue rolling); t: recessive gene (non-tongue rolling).

Prediction of the probability that a baby will be born with an inherited disease, e.g. cystic fibrosis,
page 258
, forms the basis of genetic counselling.

Co-dominance

For some traits, there can be more than two alleles that code for it, and more than one allele can be dominant. An example of this is the inheritance of A and B type antigens on the surface of red blood cells, determined clinically as the ABO system of blood grouping (
p. 60
). There are three possible alleles here: one allele codes for production of A type antigens (A), one allele codes for production of B type antigens (B) and a third allele codes for no antigen at all (O). An individual may have any combination of two of these three alleles: AA, AB, BB, AO, BO, or OO. Both A and B are dominant, and both express themselves wherever they are present. This is called co-dominance. O is recessive, and so only expresses itself in a homozygous recessive genotype. This means that individuals with an OO genotype have neither A nor B antigens on their red cell surface and are blood group O. An individual with genotype AB will have both A and B and have blood group AB. An individual with genotype AO or AA will have only A type antigens and be blood group A; someone with genotype BO or BB will have only B type antigens and be blood group B.

Sex-linked inheritance

The Y chromosome is shorter than, so carries fewer genes than, the X chromosome (
Fig. 17.2
). Traits coded for on the section of the X chromosome that has no corresponding material on the Y are said to be sex linked. The gene that codes for normal colour vision is one example, and is therefore carried on X chromosomes only. It is the dominant form of the gene. There is a rare, recessive form of this gene, which is faulty and codes for red–green colour blindness. If a female inherits a faulty copy of the gene, she is likely to have a normal gene on her other X chromosome, giving normal colour vision. A female carrying the colour blindness gene, even though she is not colour blind, may pass the faulty gene on to her children and is said to be a carrier. If the gene is abnormal in a male, he will be colour blind because, having only one X-chromosome, he has only one copy of the gene. Inheritance of colour blindness is shown in
Figure 17.10
. This illustrates the possible genetic combinations of the children of a carrier mother (one normal gene and one faulty gene) and a normal father (one normal gene).

Figure 17.10 
Inheritance of the sex-linked red–green colour blindness gene between generations.

There is a 50% chance of a son being colour blind, a 50% chance of a son having normal vision, a 50% chance of a daughter being a carrier (with normal vision herself) and a 50% chance of a daughter being normal.

Genetic basis of disease

Learning outcomes

After studying this section, you should be able to:

outline the link between cancer and cell mutation

distinguish between genetic disorders caused by gene mutation and chromosomal abnormalities, giving examples of each.

Cancer

Cancer (malignant growth of new tissue,
p. 49
) is caused by mutation (
p. 429
) of cellular DNA, causing its growth pattern to become disorganised and uncontrolled.

Inherited disease

Box 17.1
lists a number of diseases with an inherited component.

Box 17.1
Some disorders with an inherited component